1. Show up on time with all of the required materials. (If you have questions about the materials, contact the teacher a few days before the class to clarify.)
2. If you're late, apologize and then quietly try to catch up. I once took a one-day workshop where a fellow student showed up an hour late, then proceeded to complain all day about how behind she was. She even yelled at the students who used the equipment for certain steps before she did. That was two years ago, and it still makes me mad.
3. Be a student. As in, don't try to be the teacher. I recently took a class at the Art Center about PMC (precious metal clay), and a fellow student kept interrupting the teacher and giving her own mini-lectures about her experience as a silversmith. She didn't notice the rest of the students rolling their eyes.
4. Be positive. Sometimes it's frustrating to learn something new, especially if you don't get it right the first time. But you're there to learn, so keep trying.
5. Turn off your cell phone.
6. Enjoy the learning process instead of focusing on the end result. In one silver-smithing class I took, several of the students complained because they didn't like one of the projects we were doing: a silver ring. One woman said, "I would never wear this! I've really wasted my money today." My response: "No, you didn't. You're here to learn, and you did that."
I'm personally guilty of breaking the fourth rule, and I still feel badly about it. I hate the fact that I might have ruined the experience for my fellow students in that particular class (a very poorly organized book-making workshop at the Art Center). If you're out there, I'm sorry!